Particle detectors are often used to warn of the presence of smoke emanating from a potential or incipient fire. Particle detectors of the scattering light type operate by exposing an air sample, that is drawn from an area being monitored, to light, and detecting light scattered from any particles in the air. Air, for example from a factory or office, usually contains some level of particles, and the detector can be set to alarm at certain levels which are higher than background particle levels, and are believed to be indicative of smoke.
The environments that scattering type smoke detectors operate in vary widely, and include, for example, office environments, factories and manufacturing plant, power stations and clean rooms. Each of which has different levels of background particulate material.
A problem can be encountered with such smoke detection apparatus if they are continuously exposed to the relatively high levels of background pollution in the air that can exist in some environments. A large-scale example in recent years has been the high levels of smoke pollution often present in widespread regions of Asia, which have a high dependency on the burning of brown coal.
Background smoke pollution can cause soiling of components of the detector leading to premature failure, for example due to clogging of air paths or changes in the optical properties of the detection chamber itself.
Attempts to overcome this problem have included dust filters placed in the airstream. Dust filters have been used to filter out particles not associated with the smoke to be detected. Smoke particles may occur in a variety of sizes depending on the fuel used and combustion conditions, and the filter type is chosen according to type of dust particles expected and the type of smoke to be detected.
However, as conventional dust filters clog they begin to remove more particles from the air and will eventually begin filtering out smoke particles (or other small particles of interest). This may be due to effective pore size of the filter being reduced as more particles clog the filter. This can be a problem because such filters start undesirably removing smoke particles before the flow rate changes appreciably. The result is that the filter may be removing an unknown proportion of smoke, but this is not detectable using flow-meters.
In some situations attempts have been made to condition the air sample prior to its introduction into the smoke detector e.g. by diluting the sample flow with clean air. The object of such dilution is to arrive at a sample flow with an unchanged particle distribution, but with a lower particle concentration than the original sample flow.
Dilution can be used to effectively reduce the concentration of particulate material reaching the detector, but presents problems for air sampling smoke detectors that use a pipe network to draw air from a space being monitored, in that the introduction of the diluent air flow into the flow entering the detector reduces the amount of sample air drawn from the region being monitored. This causes an increase in the time taken for the sample air to travel from the region being monitored to the smoke detector, and consequently increases detection time.
One proposed dilution filter, described in U.S. Pat. No. 5,332,512 to Wells splits the sample flow into two sub-flows, and filters one of the flows to remove all particles from it. The filtered and unfiltered sample flows are then recombined.
The present inventors have ascertained that such a device would address the transport time increase identified above without requiring a substantial increase in aspirator power, however, the dilution ratio of such a device would change over time making taking reliable particle measurement difficult. More importantly the inventors have identified that the dilution ratio will increase as the capillary, through which the unfiltered air passes, clogs. Ultimately this may lead to no particles passing through the filter arrangement, which is undesirable.